Adams-Wilson Hobbycopter
And finally a pic of the Sunbury machine to boot!
Ian, I didn't recognise it at all from the much modernised version you posted along with its new name, and especially that box-section tailboom. I had hitherto only found rather poor quality pics of earlier versions known as the AW XH1 Hobbycopter but they still didn't seem to look much like the one I described.
Pleased to see it is pretty much as I remembered it now I see the mechanical layout. I wonder what happened to it.
Thanks for the solution anyway!
The thought trusting your life to that notoriously weak Sunbeam "gearbox" with chain & sprocket drive via Triumph engine, primary drive, clutch and transmission again hardly renowned for reliability makes me shudder. And no overrun clutch - the original spec talks about autorotation via a manually selected lever...what's the betting there's a Triumph clutch lever on the cyclic connected in the usual way. Some hope of achieving the autos that would be required every few minutes when something broke, stripped, melted, siezed, blew up or just fell apart!
Thankfully it doesn't seem that it was ever registered.